Conventionally, NMR analyzers are known which comprise a superconducting magnet for internally generating a powerful static magnetic field and an RF coil provided inside the superconducting magnet and which is configured so as to detect an NMR signal of a sample inserted into the RF coil (for example, Patent Document 1).
Hereinafter, a specific configuration of a conventional NMR analyzer will be described with reference to FIG. 11.
The NMR analyzer comprises an analyzer main body 100, a probe inserted into a hole 100a of the analyzer main body 100, and a test tube 105 inserted into the probe. The analyzer main body 100 comprises a superconducting magnet 101 for generating a static magnetic field in the hole 100a, and an insulating container 102 for housing the superconducting magnet 101 together with a coolant such as liquid helium. The probe comprises a magnetic field correcting member 103 having a magnetic field correcting coil 103a and a coil supporting tube 104 having an RF coil 104a fixed to the inside of the magnetic field correcting coil 103a. In addition, an NMR signal of a sample inside the test tube 105 is detected in a state where the sample is arranged inside the RF coil 104a. Moreover, a plurality of fins 105a provided on the test tube 105 are for rotating the test tube 105 around an axis thereof using air that blows out from an outlet (not shown). Due to the static magnetic field applied to the sample being averaged in a direction of rotation by such a rotational motion of the test tube 105, detection accuracy can be increased.
With the conventional NMR analyzer shown in FIG. 11, in order to analyze a plurality of samples, each sample must be placed in a different test tube 105 and the test tubes 105 must be sequentially replaced and mounted to the analyzer main body 100. To this end, the RF coil 104a is fixed to the analyzer main body 100 to enable the RF coil 104a to be commonly used for the respective test tubes 105.
Therefore, with the conventional NMR analyzer, a size of the RF coil 104a is determined based on a size of the test tube 105 and, accordingly; sizes of the magnetic field correcting member 103 and the superconducting magnet 101 (insulating container 102) which enclose the RF coil 104a are determined. This made downsizing of the entire NMR analyzer impossible. Specifically, with the conventional NMR analyzer, since the size of the RF coil 104a is set based on an assumed maximum size of the test tube 105, the size of the RF coil 104a restricts downsizing of the magnetic field correcting member 103 and the superconducting magnet 101 which enclose the RF coil 104a. As a result, there is a limit to how much the entire NMR analyzer can be downsized.
In addition, from the perspective of detection sensitivity per unit volume of a sample, the following problem is observed. Specifically, with the conventional NMR analyzer, the size of the RF coil 104a is set based on an assumed maximum size of the test tube 105. Therefore, it is difficult to improve the detection sensitivity of NMR signals when a test tube that is thinner than the assumed maximum size is used. A reason therefor will now be described with reference to FIG. 12.
FIG. 12 is a perspective view showing a widely used saddle-shaped RF coil. An upper side section and a lower side section of the saddle-shaped coil shown in FIG. 12 depict an arc along a circumferential direction of a test tube. In addition, a side connecting the upper side section and the lower side section is arranged along a longitudinal direction of the test tube. In this saddle-shaped coil, h defines a height along the longitudinal direction of the test tube and D defines a diameter of the arc depicted by the upper side section and the lower side section. An optimum geometry of a saddle-shaped coil of this type is reported in, for example, a reference (“Optimum Geometry of Saddle Shaped Coils for Generating a Uniform Magnetic Field”, The Review of Scientific Instruments, Vol. 41, Number 1, pp.122). Specifically, the reference reports an optimum geometry of the saddle-shaped coil shown in FIG. 12 of φ=120°, h=2D, where φ defines a central angle of the arc depicted by the upper side section and the lower side section of the saddle-shaped coil. In addition, with a saddle-shaped coil with an optimum geometry, it is known that a detection sensitivity of NMR signals per unit volume of a sample is proportional to B1/I that represents an RF magnetic field B1 per unit current I that flows through the RF coil. According to Expression 1 below which represents B1/I, the detection sensitivity of a saddle-shaped coil is inversely proportional to the diameter dimension D.
                                          B            1                    I                =                                            4              π                        ⁢                          μ              0                        ⁢                          h                              D                2                                      ⁢                          (                                                s                                      -                                          1                      2                                                                      +                                  s                                      -                                          3                      2                                                                                  )                        ⁢                          sin              ⁡                              (                                  60                  ⁢                  °                                )                                              =                                                    4                ⁢                                  μ                  0                                                            π                ·                D                                      ×            0.465                                              [                  Expression          ⁢                                          ⁢          1                ]            
Therefore, when there is a long distance between the RF coil 104a and the test tube 105, the detection sensitivity of NMR signals cannot be increased to an originally attainable level.
Patent Document 1: Japanese Patent Application Laid-open No. 2004-219281 (in particular, paragraph [0004])